In ribbon feeding systems for impact printers or the like, it is common to provide a ribbon drive mechanism having two spools (one takeup and one supply) each drivable by a step motor. It is also known to operate one step motor as a drag motor for applying tension to the ribbon while the other motor drives the ribbon with the two motors switching functions when the direction of the ribbon feeding is reversed. Examples of such systems may be seen by referring to U.S. Pat. No. 4,294,552 issued to J. Mako Oct. 13, 1981 and U.S. Pat. No. 4,573,645 issued to S. C. Harris on Mar. 4, 1986 and articles published in the IBM Technical Disclosure Bulletin Vol. 32, No. 6B, Nov. 1989 at pp. 430 et seq and Vol. 19, No. 11, April 1977 at pp. 4120 et seq. The underlying operative principle of such systems is that the drag motor functions as an electric generator and the feedback electrical current produced in the drag motor phase windings is applied to a resistive load to produce braking torque which resists the rotation of the supply spool by the pull of the ribbon. In U.S. Pat. No. '645, the dynamic braking of the drag motor is done by intermittently switching transistors for connecting a resistive load across the motor phase windings. The degree of torque is controlled by periodically varying the duty cycle of the load switching signals on the basis of digital duty cycle values stored in a drag look-up table applied to a chop wave generator connected to the switching transistors. The frequency of the chop wave must be high to avoid adversely affecting the use of the back emf signals from the drag motor for controlling ribbon velocity and other purposes.
Simple resistive loading across a motor phase winding produces excessive cogging which stresses the ribbon, causes it to track improperly along the print line and produces mechanical vibrations that cause noise. Additionally, switching resistive loads in and out of connection with motor phase windings at varying frequencies or duty cycles does not control the drag consistently across different rotational motor speeds. Also, frequency and pulse width modulation circuits have been seen to have resonance problems. Such techniques when managed by electronic controllers or microprocessors require more dedicated resources than desired.